1. Field of the Invention
The present invention relates to a V-belt type continuously variable transmission and, particularly to a V-belt type continuously variable transmission attached to a crankshaft of an engine via a centrifugal clutch.
2. Description of the Related Art
As V-belt type continuously variable transmissions, Japanese Patent Laid Open Application No. 2003-184972 discloses a type having a clutch function capable of cutting mechanical power transmission between a V-belt and drive pulley assembly, and Japanese Patent Laid Open Application No. 2007-71255 discloses a type in which a V-belt and a drive pulley assembly are connected so that mechanical power can be transmitted therebetween and a centrifugal clutch or the like is separately arranged between a drive shaft and a crankshaft of an engine.
In the former type, since there is no need for separately providing the centrifugal clutch or the like, it is possible to simplify the entire configuration of the mechanical power transmission system and also make the entire mechanical power transmission system more compact. However, when the clutch is disengaged, a return spring is required in order to bias a movable sheave of the drive pulley assembly to a clutch disengaged position for example. Thus, the V-belt type continuously variable transmission is increased in size and complicated. Further, only with friction force between the V-belt and the movable sheave, capacity of the clutch is limited.
FIG. 6 shows one example of the latter type. Since a centrifugal clutch 307 is arranged between a drive shaft 302 of a V-belt type continuously variable transmission 301 and a crankshaft 304 of an engine 303, the entire mechanical power transmission system is increased in size. However, since the return spring described above is not required, it is possible to simplify the structure of the V-belt type continuously variable transmission 301, and also ensure large capacity of the clutch. It should be noted that hatching for showing a section is not applied in FIG. 6.
The conventional structure in FIG. 6 will be simply described. The centrifugal clutch 307 is housed in a clutch cover 311 attached to a crankcase 310 of the engine 303, a clutch input shaft 312 is formed integrally with the crankshaft 304, and a clutch housing 314 on the clutch output side is rotatably supported on an inner surface of the clutch cover 311 through a ball bearing 315.
A transmission case assembly 320 is formed by a transmission case body 321 molded integrally with the crankcase 310 and a transmission cover 322 attached to the transmission case body 321 by bolts. This transmission cover 322 is made of resin for reducing weight. The driven shaft 330 of the V-belt type continuously variable transmission 301 is molded integrally with an input shaft 332 of a gear type transmission 331 and cantilevered on the crankcase 310. One end of drive shaft 302 is integrally coupled to the clutch housing 314 on an output side of the centrifugal clutch 307, and the other end of the drive shaft 302 is rotatably supported on a boss portion 333 made of aluminum which is a separate member from the transmission cover 322 through a bearing 334 or the like. A stay portion 333a extending along the inner surface of the transmission cover 322 is integrally formed in the boss portion 333 made of aluminum. The stay portion 333a is fixed to a cover attachment surface of the transmission case body 321 together with the transmission cover 322. Since the other end of the drive shaft 302 is supported by the boss portion 333 made of aluminum and the stay portion 333a, rigidity relative to bending stress of the drive shaft 302 is enhanced.
However, as in the conventional structure shown in FIG. 6, when the boss portion 333 made of aluminum, which is the separate member from the transmission cover 322, is provided in order to support the other end of the drive shaft 302, the number of parts is increased and assembly work of the transmission case assembly 320 is complicated.